1. Field of the Invention
This invention relates to a tester for performing functional testing on electronic and electromechanical weapon systems, including avionics systems.
2. Description of Related Art
As high technology systems, such as avionics or weapon systems, become increasingly reliant upon sophisticated electronic or electromechanical components, the number and complexity of test procedures required to provide support for the systems also increases. Automated test equipment has been developed which enables an organization to achieve organic support of a system with relatively low-skill level personnel, but as the system becomes more complex, the conventional, sequential, parametric test programs used by automated test equipment tend to grow in length. The escalating costs of automatic test equipment and conventional test programs, coupled with test times measured in hours, has made this method of systems support increasingly less desirable in a variety of contexts.
An early system adopted for use in the context of avionics was the Versatile Avionics Shop Tester (VAST) developed to test avionics for aircraft built by Grumman Corporation. VAST was a complex and sophisticated system and the Navy's first attempt at a general-purpose test station. The physically large system consisted of nine racks filled with electronic instruments ranging from a digital subsystem to a cynchro-resolver standard. High power consumption and heat dissipation required the use of forced-air cooling.
Test program sets developed on VAST tended to have long run times and complex fault isolation paths. The programs often involved rigorous testing that, in hindsight, seems inappropriate for the application. Power supply outputs, for example, were often tested to extremely tight tolerances even for parameters such as long-term drift and AC ripple. The "unit under test" (UUT) parameters were frequently checked for accuracies to which they were designed, but beyond their functional use. Some of these tests were appropriate for an "acceptance" test procedure, in which a design is tested for specification adherence.
"Field" testing differs significantly from acceptance testing. Field problems are not likely to be slight losses of tolerances, but rather complete failures. Units removed from an aircraft for a power supply problem, for example, are far more likely to be due to complete loss of power supply output than to an output that exceeds 1% of nominal. Avionics units are removed from aircraft during organizational level maintenance because they have hard failures, or perhaps because they have intermittent failures, but not because they are merely operating outside of specification.
The type of testing in which a unit is tested to its design parameters is called "parametric" testing. Parametric testing is typified by a high degree of testing coverage, possibly excessive in some cases; by generally sequential application of stimuli and evaluation of responses; extensive fault isolation limited only be test connector access; and comparatively long test times.
Parametric testing can sometimes indicate a problem where no real problem exists. For example, a unit might be tested solely because it has just been received or taken out of inventory and the maintenance chief must determine if it is ready for issue. The UUT might fail the test program because an output is being tested to a tolerance beyond its functional use. While in the aircraft, the same unit would appear to be operational, and the on-board built-in test (BIT) would indicate no problem.
An unnecessary and undesirable conflict is created in this situation, in which the customer caught in the middle with either be suspicious of the unit or, more likely, lose confidence in the test program. In that case, the test program will fall into disuse, and the lack of confidence might spread to include other programs. Although such a scenario should not imply that a test program should be cursory or superficial, it does become clear that an intelligent compromise must be reached.
The concept of functional testing is a response to these testing enigmas. Functional tests check a UUT in accordance with its functional use, rather than to rigorous criteria to which the unit was designed and accepted. Functional tests are based on tolerances that provide satisfactory operation in an aircraft or other weapon system. Functional tests take significantly less time to run than parametric tests and make greater use of, and are more reliant on, a unit under test's internal-test circuitry and software. A functional confidence test of a UUT should take no more than half an hour to execute.
Units under test generally possess internal microprocessors that, when so instructed, run designed-in diagnostic BIT programs. The BIT hardware and software designed into a unit can be analyzed to determine which circuits are not checked by BIT so that supplemental testing can be added to the program as necessary.
The majority of new weapon systems have improved BIT programs and hardware, and are designed around a standard bus architecture. Conventional automatic test equipment hardware and test programs are not designed to take full advantage of the weapon system's improvements. Functional testing, on the other hand, provides a cost-effective means for support of modern electronic and electromechanical weapon systems. It is user-friendly, inexpensive, and requires minimal operator training and employs moderate-cost test programs that provide short test times. The tester must be capable of fault detection or fault isolation of either "black boxes" or electronic modules from a wide range of weapon systems.
Essentially, the difference between conventional testing and the proposed functional approach is that classic automatic testing performs fixed-sequence parametric tests requiring many high-precision instruments and long test times, while functioning testing employs variable-sequence functional test programs that take advantage of all available information including input/output activity, while running the BIT program of the UUT. This approach requires fewer high-precision instructions and shortens test times.
Recently, a high performance, relatively compact type of classic automatic test equipment known as the Screening or Small Computerized Automatic Tester (SCAT) has been develop which incorporates a number of hardware and software advances, although in the context of parametric rather than functional testing. Key features of SCAT are that it is contained within a package which is easily transportable and uses a number of innovative design concepts including smart switch routing via a high-performance backplane bus, test program storage using plug-in bubble memory cartridges, and high-performance instruments-on-a-card which are each compatible with the system backplane and whose configuration may therefore be varied simply by installing different cards for each type of system to be tested. The test programs stored in bubble memory use Department of Defense mandated standard test language IEEE-716 ATLAS (Atlas) and a mainframe compatible operating system. An example of a SCAT system is disclosed in U.S. Pat. No. 4,760,329, assigned to the present assignee, and incorporated herein by reference.
The heart of the SCAT system is the family of high performance instruments-on-a-card that plug into the industry standard IEEE-796 bus. The instruments-on-a-card may include a digital multimeter, a counter/timer, a function generator, and so forth. Each SCAT instrument is self-contained on a single plug-in card. This highly modular architecture offers several important advantages. The tester is easily customized to meet unique analog, digital, or hybrid requirements, and is easy to upgrade to meet next-generation test requirements by adding new instruments incorporating the latest technology available.
The present invention uses many of the advances embodied by SCAT, including the instruments-on-a-card concept and a high-performance backplane, bus-oriented architecture, but adapts the system for use as a function tester rather than as a parametric tester.